1. Field of the Invention:
The present invention relates to a heat exchanging unit, especially to a kettle-type heat exchanging unit used for precooling in natural gas liquefaction, purification in ethylene production, LPG (Liquefied Petroleum Gas) recovery from natural gas, etc., which uses the latent heat of evaporation of a cooling medium. The present invention further relates to a heat exchanging unit in which it is possible to reduce the size of the suction drum disposed upstream of a compressor used for separating mist from a gasified cooling medium generated from using of the latent heat of evaporation of the cooling medium.
2. Description of the Related Art:
For natural gas liquefaction, ethylene purification, LPG recovery, etc. by low temperature processing, a kettle-type heat exchanger is used. The conventional kettle-type heat exchanger includes a closed vessel having a cooling medium liquid retaining zone, a cooling medium gas holding zone located thereabove, and a through pipe through which fluid to be cooled is passed through the cooling medium liquid retaining zone of the closed vessel.
When the cooling medium liquid is charged in the closed vessel through an expansion valve, it expands under reduced pressure to gasify partly and its liquid temperature is lowered to a boiling point temperature corresponding to the pressure inside the closed vessel.
The resulting low temperature cooling medium liquid in the cooling medium liquid retaining zone cools the fluid to be cooled flowing through the through pipe, and gasifies partly. The resultant gas moves to the cooling medium gas holding zone.
A number of these kettle-type heat exchangers are so connected in a multistage system so as to progressively decrease the pressure and the boiling point of the cooling medium, thereby gradually lowering the temperature of the fluid to be cooled.
The cooling medium gas retained in the cooling medium gas holding zone includes a mist formed during boiling of the cooling medium liquid. This cooling medium gas is generated from contact of the cooling medium liquid with the heat exchanger, and is thereafter it is compressed, liquefied and recycled for use as a cooling medium liquid. The cooling medium mist included with the cooling medium gas discharged from the heat exchanger must be separated prior to the compression of the cooling medium gas. For this purpose, a suction drum having a built-in demister is provided upstream of the compressor.
Conventionally, to suppress the entrained cooling medium mist discharged from a kettle-type heat exchanger, reduction of the flow rate of the cooling medium gas has been performed by enlarging the cooling medium gas holding zone of the heat exchanger or expanding an outlet nozzle of the cooling medium gas. Accordingly, enlargement of the shell diameter of kettle-type heat exchanger, the diameter of suction drum, and the diameter of connection pipes have been inevitable.